\documentclass[tikz,convert=pdf2svg]{standalone}
\usepackage{tikz}
\begin{document}
    \begin{tikzpicture}
    [every node/.style={fill=blue!30,draw=blue!70,rounded corners},
     edge from parent/.style={blue,thick,draw}]
        \node {root}
            child {node {a1}}
            child {node {a2}
                child {node {b1}}
                child {node {b2}}}
            child {node {a3}};
    \end{tikzpicture}
\end{document}